Symmetry is mirroring on wrong axis
This is because Blender assumes the model is symmetrical in its local space and that would be very useful if you had the model's rotation match it's local space and if it's also rotated in the scene, because the symmetrical sculpting would still work on the object, however for some reason you have the model's space rotated but not the mesh. Maybe you rotated it's origin point accidently.
Select the model in the viewport in object mode and apply rotation data with Ctrl+a -&gt; Rotation so that the orientation of the objects local space is forced to match the global space: